Which application on my computer is using the network?

I remember a program called little snitch. I used it before but I realized it was just a couple programs calling home every now and then

 

Yeah, I looked at Little Snitch but it doesn't appear to do what the OP is looking for. It just tells you if some strange program tries to access the Internet and lets you block it, which is definitely a nice feature.

 

lsof -i

Only instantaneous report. No rates.

 

I have a free program called iStat Menu's that gives you a bunch of different options of stuff to display on the menu bar like temp, fan speed, network, bluetooth, time/date, and others I can't remember. You don't have to have them all showing though, you can choose which ones you want to see, so you could pick just network.